One of the processes in manufacture of semiconductors is application of a coating solution on a substrate such as a semiconductor wafer to form a coating film thereon as an interlayer insulating film or a device-protection film.
In detail, a coating solution containing polyimide and solvent is applied on a substrate or wafer, thus a coating film made of polyimide being formed after evaporation of solvent.
It is preferable to dry a coating-solution-applied wafer in a low-pressure dryer to dry the wafer for a short period. This is because a low volatile solvent such as thinner having a high boiling point is mostly used as the solvent contained in a coating solution.
Shown in FIG. 1 is a known low-pressure dryer equipped with an airtight chamber 1 with a cover 11 and a table 12. Provided on top of the cover 11 is an exhaust opening 13 connected to a vacuum pump 15 thorough an exhaust passage such as a pipe 14, for decompressing the chamber 1. Further provided in the chamber 1 is an elevatable diffuser plate 16 for forming a uniform exhaust stream over a wafer W. The exhaust stream is composed of evaporating components of a coating solution applied on the wafer W.
In the known low-pressure dryer, a coating-solution-applied wafer W is placed on the table 12. The diffuser plate 16 is then adjusted for its height so as to face the wafer W.
The wafer W is heated to a specific temperature such as 30° C. by a temperature adjuster (not shown) provided in the table 12. The vacuum pump 15 is then operated to decompress the chamber 1, to promote evaporation (drying) of the solvent in the coating solution, evaporating solvent components forming an exhaust stream spreading outwardly from between the wafer W and the diffuser plate 16. Polyimide components remaining on the wafer W is turned into a coating film thereon.
Discussed below are several drawbacks of such known drying mechanism.
Firstly, a solvent evaporation speed could depend on the difference in density of solvent contained in a coating solution and gaseous solvent near the surface of the coating solution. The larger the density difference, the higher the evaporation speed.
Solvent components evaporating from the coating solution will be formed into a stream flowing outwardly along the wafer surface with the help of the diffuser plate 16 and discharged from the chamber 1.
In detail, the evaporating solvent components will be discharged upright while being attracted in all directions from a center region of, for example, 40 mm in radius from the wafer center. The density of the gaseous solvent will thus be kept relatively low, thus evaporation of the solvent being promoted.
In contrast, the density of the gaseous solvent on the outer region of the wafer W will be high due to combination of the evaporating solvent components from the coating solution on the outer and center regions, thus evaporation of the solvent being delayed.
In summary, as shown in FIG. 2, the evaporation speed on the wafer center region will be high whereas that on the outer region will be low, thus causing variation in evaporation over the wafer surface.
Moreover, the surface temperature of the coating solution will be decreased due to heat of evaporation while the solvent is evaporating from the coating solution. The variation in evaporation speed discussed above will cause temperature difference of about 2 to 4° C. between the coating solution on the wafer center region and the outer region, or the temperature of the coating solution on the wafer center region (evaporation promoted) will be lowered quickly due to heat of evaporation compared to the outer region. The coating solution over the wafer W will thus be attracted to the center region with the help of surface tension. This could result in a thick or swelling coating film formed on the wafer center region, as illustrated in FIG. 3.
Moreover, the film of coating solution formed on the outer region of, for example, 5 mm in width from the wafer edge, will be round as illustrated in FIG. 4A, due to surface tension caused by adherence of the coating solution to the wafer surface and condensation of the coating solution.
Low-pressure drying to the wafer W having such coating solution formed on the outer region in the known Low-pressure dryer shown in FIG. 1 could promote evaporation of solvent from the round section in addition to upward evaporation from the outer region.
In detail, the coating solution on the outer region having the evaporation area larger than the inner region will be evaporated much more than that on the inner region. Evaporation of solvent from a coating solution will cause temperature decrease on the solution surface due to heat of evaporation, thus producing a large surface tension.
The differences in amount of evaporation and surface tension due to the difference in evaporation area between the outer region and the inner region will force the coating solution to be attracted to the outer region. This could result in a thick or swelling coating film formed on the wafer outer region, as illustrated in FIG. 4B, in contrast to that shown in FIG. 3.